 <!DOCTYPE html>
 <html lang="en">
 <head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title></title>
  <style>
    .content{
      width: 1100px;
      height: 500px;
      background-color: orange;
      margin: 0 auto;
    }
    .item{
      width: 221px;
      height: 100px;
      background-color: yellow;
      margin-right: -1px;
      float: left;
      box-sizing: border-box;
      border: 1px solid black;
    }
    .box{
      /* 没有设置宽度,默认是auto对应父级是1190px */
      /* 父级盒子的宽度=子盒子的宽度+ml+mr */
      /* 这时候这个盒子的宽度是1200px */
      margin-left: -1px;
      /* background: white; */
      height: 200px;
    }
  </style>
 </head>
 <body>

  
  <div class="content">
    <div class="box">  
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
   </div>
  </div>
 </body>
 </html>